Pulsed ion source



Oct. 14, 1958 2,856,532

EQ F. MARTINA PULSED ION SOURCE Filed June 16, 1955 ATTQRNEY.

United States Patent Oce 2,856,532 Patented Oct. 14, 1958 PULSED IONSOURCE Eugene F. Martina, Castro Valley, Calif., assigner to the UnitedStates of America as represented by the United States Atomic EnergyCommission Application June `16, 1955, Serial No. 516,056

`t Claims. (Cl. Z50-49.5)

The present invention relates to an improved ion source and, moreparticularly, to an ion source adapted to produce a pulsed beam of ions.

Pulsed ion sources Athat employ a constant stream of volatilized sourcematerial experience various difficulties in handling of the sourcematerial in that an overly large quantity thereof is expended without aproportionate ion current and there is produced contamination of thesource and attached apparatus utilizing the ion beam by quantities ofun-ionized source material liberated between pulses of sourceenergization. Attempts to supply from such as a 'bottle of volatilizedsource material pulses of gas to be ionized have proven generallyunsatisfactory because of the ditlculty in properly and accuratelytiming the tlow of gas.

The present invention satisfies the criteria for pulsed ion sourceswithout `the diiculties attendant other known pulsed ion sources and inaddition provides a source having a very long life by the accomplishmentof the following objects:

It is an object of the present invention to provide an improved pulsedion source having `a long term source material supply.

It is an object of the present invention to provide an improved pulsedion source having an occluded gas supply.

It is another object-of the present invention to provide an improvedpulsed ion source having ionizing means and a gas `supply occluded in`an auxiliary lament not cornprising one of `the ionizing electrodes.

Numerous other more specific objects and advantages of the inventionwill become apparent to those skilled in the art from the -followingdescription of a preferred ernbodiment of the invention taken togetherwith the accompanying drawing wherein the sole figure is a schematicsectional View of the ion source together with appropriate associatedelectrical circuitry.

Considering the structural details of the illustrated embodiment of theinvention and referring to the drawing, the source proper will be seento be contained in an insulating cylindrical shell 11 having a groundplate 12 secured thereto in closing relation to one end and an end plate13 secured in closing relation to the other cylinder end. Joinder oftheshell 11 and plates 12and 13 thereto may be facilitated `by theprovision of flanges about the cylindrical shell ends against which theplates 12 and 13 are tightly drawn as by bolts 14 and these joints aremade vacuum tight by the insertion of suitable gaskets 16 therein. Apipe 17 is secured to the shell 11 in sealed communication with theinterior thereof and is connected to evacuation means, such as one `ormore vacuum pumps 18 whereby the shell may be continuously evacuated.

Within :the cylinder shell there is disposed a first cylindricalelectrode 21 secured to the ground plate 12 and extending coaxially `ofthe shell toward the opposite shell end past the center `of the shell. Asimilar cylindrical `electrode 22 `of greater `diameter than the firstcylinder 21 is mounted upon the `inner surface of the end plate 13 andextends coaxially of the shell 11 toward the ground plate in overlappingrelation to the free end of the rst cylinder 21 and out of contacttherewith. Within the cylinder 21 and adjacent the open end thereof isdisposed a grid electrode 23 having an open mesh structure and disposedin a plane normal to the axis of the cylinder 21. This electrode ismounted in insulated relation to the ground plate 12 and cylinder 21 asby means of one or more insulating standards 24 affixed to the groundplate in rigid relation thereto. A lead-through insulator 26 in theground plate extends an electrical lead from the grid 23 to the exteriorof the shell for connection to suitable circuitry described below.

An electron emissive filament 27 is also provided within the shell 11 inspaced relation to the grid 23 toward the ground plate 12 therefrom, andthis filament 27 may take the form of a coil or of a flat-wound spiralof electron emissive conductor. The filament is mounted in xed positionby one or more insulating supports 23 extending longitudinally inward ofthe shell from the ground plate V12 with a lead-through insulator 29extending through the4 ground plate and Aproviding an externalelectrical terminal connected by a lead through a support 28 to thefilament 27, the other end of which is grounded.

A gas source is provided within the shell for the ionizing means thereinand this gas supply includes a wire or bar of metal 31 characterized byan ability to absorb or occlude large amounts of gas. Where hydrogen orisotopes thereof is to be ionized metals such as titanium, tantalum,palladium, and zirconium, for example, have been found to be suitable.The wire 31 is prepared by outgassing at high temperatures to removerandom gas 'therefrom followed by exposure of the metal to the sourcegas for absorption at high temperatures and a subsequent sloW-cooling ofthe metal with a resultant retention by the metal of a large quantity ofsource gas. The wire 151 is supported adjacent the filament 27 andseparated from the grid 23 thereby between a pair of Vinsulators 32extending inward of the shell from the ground plate 12. One end of thewire 31 is grounded as by connection to the ground plate 12 and theother wire end is relectrically connected through an insulator 32 to aleadthrough insulator 33 in the plate 12 for connection externally ofthe `shell to appropriate energizing means. The gas source is laterallysurrounded by a metal cylindrical shield 3ft mounted upon the groundplate 12 and extending beyond the `filament interiorly of the shell.

Ions formed within the above-described source may be utilized therein ormay be employed exteriorly of the shell 11 and, in the latter instance,ion egress means such as an aperture or window 36 is provided in the endplate 13 in axial alignment with the grid 23 and filament 27.

Energization of the above-described elements of the ion source may beaccomplished by means of appropriate circuitry vdisposed exteriorly ofthe source proper. There is illustrated in the drawing a suitablecircuit including a power supply 51 connected between the lilamentterminal 29 and ground for passing a heating current through thefilament to heat same to incandescence whereby electrons are thermallyemitted therefrom. Pulsed energization of the gas source is accomplishedby a direct current power supply 52 which has the negative terminalthereof grounded and the positive terminal connected through a resistorto the anode of a grounded gaseous discharge tube 53. The anode of tube53 is connected to one side of a capacitor 54, the other side of whichis joined to the terminal 33, and the control electrode of the tube 53is connected to the output of a pulse generator 56. The tube 53 isnormally nonconducting and conducts Vin response to voltage pulsesimpressed electron discharge.

3 upon the tube control electrode from the pulse generator 56 so thatthe capacitor 54 discharges through the gas supply member 3l toinstantaneously heat same and drive Aoff vapor or gas occluded therein.The capacitor 54 reterminal 26 and the positive terminal of the powersup' ply 57. A direct current power supply 59 has the positive terminalconnected through a dropping resistor 6l to one side of a capacitor 62that has its other side grounded through the primary winding of thetransformer S. The capacitor 62 is discharged through the transformer bymeans of a gaseous discharge tube 63 having a grounded cathode and ananode connected to the power supply side of the capacitor. The tube 63is normally nonconducting so that the capacitor 62 is charged from thepower supply 59 through the resistor 61 and the tube is triggered bypulsed signals applied to the control electrode of the tube from thepulse generator 56 through a delay line 64 having a predetermined timedelay. Conduction of the tube 63 discharges the capacitor 62 through theprimary winding of the transformer 5S so that there is induced a voltagein the secondary winding thereof to supply an instantaneous negativegrid voltage.

The end wall 13 and electrode 22 attached thereto are likewise energizedfrom a grounded direct current power supply 66 having a positiveterminal connected through a dropping resistor 67 to one side of acapacitor 68 that has the other side thereof grounded through theprimary winding of al transformer 69. The secondary winding of thetransformer 69 is grounded at one end and connected at the other to theion source end wall 13. The capacitor 68 is discharged by means of agaseous electric discharge tube 71 having a grounded cathode and ananode connected to the capacitor 68 on the opposite side thereof fromthe transformer 69. Therv tube 71 is normally nonconducting, duringwhich time the capacitor 68 charges and remains charged, and a positivesignal on the control electrode of the tube triggers same to dischargethe capacitor 68 and applies a negative voltage pulse through thetransformer 69 to the end plate 13. The tube control electrode isconnected through a delay line 72 to the delay line 64 attached to theoutput of the pulse generator 56 so that the pulse to tube 71 is delayeda predetermined period beyond the signal applied to the grid 23 of theion source.

Considering now the operation of the improved ion source of the presentinvention and assuming the shell 11 to be evacuated, the power supply S1will be seen to provide a current flow through the filament 27 wherebythe latter is maintained electron emissive. The power supply 57maintains the grid electrode 23 at a positive potential with respect toground and thus also with respect to the filament 27 so that electronsare attracted from the filament toward the grid to form a continuous Asthe remainder of the source including the cylinders 21 and 22 arenormally at ground .potential this electron discharge yextends inconcentrated form only in the general region of the filament-grid space.

ulsed production of ions within the source is accomplished by actuatingthe pulse generator 56 to produce a voltage pulse which is applied tothe control electrode of the discharge tube 53 to cause conduction ofthis tube. As the tube 53 conducts, the capacitor 54 discharges -toapply a current pulse through the terminal 33 to the gas source member31 whereby same is momentarily heated to drive off gas or vapor occludedtherein. As

the capacitor S4 discharges the anode potential of the tube 53 falls sothat the tube is rendered nonconducting until the next triggering pulseat the control electrode. rl`he gas or vapor driven oft` the member 31drifts into the filament-grid space by virtue of the vacuum pump causinga progressively better vacuum from the ends to the center of the shell11 and this gas is ionized by the electron discharge from the filamentto produce an arc discharge. The signal from the pulse generator whichwas applied to the tube S3 is also applied to trigger the tube 63 aftera predetermined delay in line 64 so that the tube 63 fires and applies asignal across the transformer 58 that appears as a negative signal inthe secondary circuit thereof to momentarily drive the grid electrode 23negative with respect to ground. As the capacitor 62 discharges, thetube 63 is rendered nonconducting; however, the negative grid pulsewhich follows ionization of the gas in the electron discharge attractspositive ions from the arc and accelerates same to the right in thedrawing through the grid 23.

Similarly the pulse signal from generator 56 is likewise applied to thetube 71 through the delay lines 64 and '72 so as to render the tube 71conducting at a time following attraction of ions from the arc. As thetube "Il conducts it passes a pulse signal through the transformer 69which appears as a negative pulsed voltage at the cylinder 22 and endwall 13 whereby ions passing through the grid electrode 23 are subjectedto the attractive force thereof and forcibly driven through the cylinder22 toward the end wall 13 where they may pass through the window 36 orbe otherwise employed as desired.

What is claimed is:

l. An ion source comprising an electron emissive filament energized forelectron emission, a grid electrode disposed in spaced relation to saidfilament and normally maintained at a positive potential with respectthereto for establishing an electron discharge in a zone between saidfilament and said grid, a member having a gas occluded therein disposedadjacent said discharge zone and out of contact therewith, said memberbeing adapted for pulsed heating to drive pulses of gas therefrom intosaid discharge zone where same is ionized, and said grid being adaptedfor pulsed energization to establish a positive electric fieldattracting ions from said zone through said grid.

2. An ion source comprising an electron emissive filament, a griddisposed adjacent said filament, a power supply maintaining said gridnormally positive with respect to said filament whereby electrondischarge occurs between said filament and grid, an electricallyconducting member having gas occluded therein disposed immediatelyadjacent said filament on the opposite side thereof from said grid andadapted for electric pulsed heating to drive gas therefrom, meansdirecting said gas into said discharge for ionizing same, and meanselectrically pulsing said grid at a negative potential for attractingions from said discharge.

3. An ion source as claimed in claim 2 further defined by meanscontrolling pulsed heating of said member and negative energization ofsaid grid for delaying the latter a predetermined time following theformer whereby said grid attracts ions only during the period same areformed in said discharge.'

4. An ion source comprising an electron emissive filament energized toemit electrons, a grid electrode spaced from said filament and normallymaintained at a positive potential with respect thereto whereby anelectron discharge occurs therebetween, a member having gas occludedtherein disposed adjacent said discharge and out of contact therewith,means for passing a pulsed heating current through said member fordriving bursts of gas therefrom, means directing said gas into saiddischarge for ionization thereof, means electrically pulsing said grid apredetermined time following heating of said member for establishing apulsed electric field about said grid attracting ions from saiddischarge, and an electrode disposed on the opposite side of said gridfrom said filament and electrically pulsed a predetermined timefollowing grid pulsing for attracting ionsv from said grid.

5. An ion source for producing a pulsed beam of ions comprising meansestablishing a substantially continuous electron discharge through anevacuated discharge zone, an electrically conducting member containinggas occluded therein disposed adjacent said zone exteriorly thereof,means electrically energizing said conducting member in a pulsed fashionto heat same and drive therefrom bursts of gas which drift into saidzone for ionization, and electrode means periodically energized toattract ions from said zone.

6 An ion source comprising an envelope having an evacuation openingtherein, means within said envelope energized to establish asubstantially constant electron References Cited in the iile of thispatent UNITED STATES PATENTS 2,316,276 Motz Apr. 13, 1943 2,507,652Smith May 16, 1950 2,507,653 Smith May 16, 1950 2,528,547 Reilly et al.Nov. 7, 1950 2,694,151 Berry Nov. 9, 1954 2,766,397 Nienhuis Oct. 9,1956

